1. Technical Field
This invention relates to the technology of making multiple core assemblies, and more particularly to bonding surfaces of such cores to each other to eliminate flash therebetween as a result of casting.
2. Discussion of the Prior Art
Intermated sand cores are useful in large assemblies such as in automotive engine blocks and heads. Such cores are usually glued or pasted together, not locked by shaping, because multi-faceted cores with compound draft angles are difficult or impossible to form. Such pastes work well with sand cores constituted of 98% sand (Sio.sub.2) and approximately 2% bonding agent. Conventional drying of such pasted cores, in a dry oven, causes poor casting quality, such as jacket separation in an engine block or plating or flash in waterjacket passages. This is caused by the fact that the core paste will dry from the outside. The paste will form a hard skin over the outer surface, inducing the paste to generate air bubbles therein; the paste will swell or expand as the paste dries further forcing the jackets to separate, resulting in major quality problems suggested above.
Drying of mold components by means other than conventional ovens, such as with microwave energy, has been used, but limited to refractory materials in cores with surface bonding materials. U.S. patents disclosing the use of microwave energy to dry core sand mixtures containing resins, include U.S. Pat. Nos. 4,763,720 and 4,331,197. U.S. Pat. No. 4,655,276 discloses the use of microwave energy to dry a refractory ceramic coating on a sand core applied in a slurry thickness in excess of 0.01 inch. It would not be readily suggestive to those skilled in the art to use microwave energy for drying wet pastes having a thickness in the range of 0.005-0.020 inches because of the fear of potential boiling of ultra-thin coatings and the resulting nonuniformity of the placement of the cores in the bonded condition.